Anticipation logic for a surge control valve utilized with load compressor

ABSTRACT

A load compressor includes an air inlet and an outlet, and a surge control valve positioned on the outlet. The surge control valve serves to selectively pass air to a downstream use, or to be dumped to atmosphere. A control for the compressor and the surge control valve is also included. The control is provided with a variable indicative of the operation of the load compressor, the variable being indicative of a movement to transient operation of the compressor. The control is provided with a low surge control limit for steady state operation. The control is also provided with a higher surge control limit, and switches to use of the higher surge control limit if the variable being monitored is seen to change beyond a particular threshold. The low and higher surge control limits are utilized to control the opening of the surge control valve. An aircraft air supply system and a method are also disclosed.

BACKGROUND

This application relates generally to a method to optimize the operation of a compressor which delivers pneumatic power in order to maximize performance and maintaining proper stability margin for both steady state and transient operation.

An APU is a small gas turbine engine that provides a number of functions associated with an aircraft other than propulsion, particularly before the main aircraft engines are started. Some APU configurations include a shaft-mounted load compressor which delivers pneumatic power to aircraft accessories.

Load compressor bleed air is delivered to aircraft systems through a bleed control valve. Some of the air may be utilized as cabin air before the main gas turbine engines are started, and the load compressor may also supply air to the main gas turbine engines at start-up.

Normally, the APU-supplied accessories have a wide range of compressed air requirements, which the APU bleed control valve has to accommodate. If the air demand drops below certain limits an unstable operation on the compressor, called “surge” may occur.

To maintain the compressor flow above its surge level an anti-surge valve is used. The engine is programmed with anti-surge logic to modulate the anti-surge valve to divert a portion of the compressor flow to the atmosphere and therefore to maintain the compressed air flow above minimum limits to avoid surge.

The compressor minimum air flow limits are set taking into account numerous parameters including operating conditions, compressor inlet guided vanes position, bleed demands, etc. Another important aspect which is considered is the operation during transient maneuvers. During a fast reduction of the air flow requirement, the compressor minimum air flow could drop momentarily below the minimum limit and cause surge.

Thus, in order to prevent such situations and maintain a proper operation of the compressor, the minimum bleed flow limit has to be raised. A higher minimum bleed flow limit reduces in certain operating conditions the APU performance, increasing the engine temperature and fuel consumption and, in the same time, reducing the bleed air quality, lower bleed pressure and flow.

SUMMARY

A load compressor includes an air inlet and an outlet, a bleed valve and an anti-surge valve positioned on the outlet. The bleed valve serves to selectively pass air to a downstream use while the anti-surge valve dumps it to atmosphere. A control for the compressor and the surge control valve is also included. The control is provided with a variable indicative of the operation of the load compressor, the variable being indicative of movement to transient operation of the compressor. The control is provided with a low surge control limit for steady state operation, and a higher surge control limit The low and higher surge control limits are utilized to control the opening of the surge control valve.

An aircraft air supply system and a method are also disclosed.

These and other features of the present invention can be best understood from the following specification and drawings, of which the following is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically shows a load compressor for an aircraft.

FIG. 1B shows another embodiment.

FIG. 2 shows operating conditions.

FIG. 3 is a flowchart of an inventive logic.

DETAILED DESCRIPTION

FIG. 1A schematically shows a load compressor 24, APU 22, and aircraft use 32 as a combined system 20. APU 22 typically includes a small gas turbine engine that provides electricity, but also drives load compressor 24. The load compressor 24 has an air inlet 26, and delivers the air through an outlet 21, and through a combined surge control and bleed valve 28 to a downstream use 32. The downstream use is typically associated with an aircraft, and may include the aircraft passenger cabin, as well as the main gas turbine engines on the aircraft.

A controller 29 controls compressor 24 and the valve 28. When the surge control valve is open, it directs a portion of the compressed air to a dump 30 where the compressed air is returned to atmosphere. Other air is directed into line 34 and to use 32. So-called “fast-acting” valves may be utilized for surge control valve 28.

The valve 28 is a valve which can divert a portion of the flow to the dump line 30, and another portion to line 34. When a surge limit is exceeded, as will be explained below, the control 29 controls the valve 28 to divert at least a portion of the air to the dump line 30.

FIG. 1B shows an embodiment 120 which is somewhat similar to the FIG. 1A embodiment. Each of the elements which are identical to the FIG. 1A embodiment are included with 100 added to the reference number. As such, the control 129 works in combination with the load compressor 124. The load compressor 124 receives air from inlet 126, and delivers it through outlet 121 to a downstream use 132. The APU 122 drives the load compressor 124. The use 132 receives air from a line 134. What is different in the FIG. 1B embodiment is the single valve has been replaced with a bleed valve 131, and an anti-surge valve 128. The two valves were combined in the valve 28 of FIG. 1A. The anti-surge valve 128 is controlled to open and dump air to atmosphere 130 should the surge limits be exceeded, as will be explained below. When closed, the anti-surge valve 128 allows air to be passed from the outlet of load compressor 124 to downstream use 132 as further limited by bleed valve 131.

The prior art control of a phenomenon known as surge margin can be described by the operating conditions shown in FIG. 2. The line X shows operational points for a typical load compressor. A first higher surge control limit is shown at Y. A second lower surge control limit is shown at Z. While the load compressor is operating at one operating point, T₁ for example, there is no problem with surge. However, as the illustrated load compressor dynamic head measurement LDC DPP drops, the operation approaches a point T₂. Existing pressure sensors located in a load compressor diffuser and another in the load compressor exit may be used. The difference between the two pressures would be indicative of the LDC DPP. Of course, other ways of sensing the pressure may be used. The LDC DPP is generally inversely proportional to the discharge pressure from the compressor. T₂ is generally shown at the upper surge control limit Y. Another point, T₃ is also illustrated.

To provide the most efficient performance at steady state operation, a lower surge control limit such as Z is desirable.

On the other hand, this limit may not be acceptable for fast transient operations, such as startup of a main gas turbine engine on an aircraft receiving APU 22/122.

The present invention provides a control logic which anticipates a movement to transient operation, and a need for the higher limit.

Some variable is observed and a change in this variable would be indicative of a movement towards surge. Since the illustrated variable LDC DPP is proportional to flow (and inversely proportional to pressure), a drop in the LDC DPP, or a negative derivative or slope would be indicative of the movement towards surge.

Thus, as shown in FIG. 3, a control logic, which may be included in control 29, 129, looks for change in the variable. If the change exceeds a limit, then the higher surge control limit is utilized. On the other hand, if the change does not exceed the limit, then the lower surge control limit is utilized.

Looking at the change in the variable provides very prompt response such that the system will operate on the higher transient surge control limit extremely quickly once a transient condition has begun.

In one embodiment, a threshold for the change in LDC DPP may be if the rate of change is below −2% per second, for example. In some embodiments the threshold limit may be function of engine speed, inlet guide vane position or other operating condition. Further, the logic may require that there be consecutive readings below this threshold. For example, readings could be taken every 20 millisecond, and only when a particular number of consecutive readings, three for example, are below this threshold would trigger the surge control limit be changed from the steady state low value to the high transient value. Of course, the numeric values are purely examples. Any other rate of change, timing between readings, or number of consecutive readings can be utilized.

The system will return to the lower steady state limit once the amount of change is reduced such that a new steady state condition appears to have been reached.

Switching from a lower surge control limit to a higher surge control limit works slightly different depending where the compressor operating point was prior to the occurrence of the fast reduction of flow requirement:

a. If the compressor operating point is above limit Y (e.g., point T₁), the anti-surge control logic may overcome the time lag of anti-surge valve control and operation by commencing the valve opening before the steady state limit is reached.

b. If the compressor operating is below limit Y (e.g., point T₃) the anti-surge control logic will detect a high delta between the compressor operating point and the transient limit (limit Y). The control can generate a higher opening command to accelerate the valve opening. As an example, the signal to the valve may be increased proportionally based upon the magnitude of the error.

By allowing a lower surge control limit (e.g., limit Z) under certain operating conditions, APU performance may be improved and fuel consumption reduced. While a load compressor operating with an APU is disclosed, the features of the application may be beneficial in other types of compressors.

Although embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention. 

1. A compressor system comprising: a compressor having an air inlet and an outlet, a surge control valve positioned on said outlet, said surge control valve serving to selectively pass air to a downstream use, or to be dumped; and a control for said compressor and said surge control valve provided with a variable indicative of movement of said compressor to transient operation, said control further being provided with a low surge control limit, and said control being provided with a higher surge control limit, said control switching to use of said higher surge control limit if the variable being monitored is seen to change beyond a particular threshold, and said low and higher surge control limits being utilized to control the opening of said surge control valve.
 2. The compressor system as set forth in claim 1, wherein said compressor is a load compressor in an auxiliary power unit.
 3. The compressor system as set forth in claim 1, wherein said variable is a dynamic head pressure of said compressor.
 4. The compressor system as set forth in claim 3, wherein said dynamic head pressure is inversely proportional to a discharge pressure of said compressor.
 5. The compressor system as set forth in claim 1, wherein said change in said variable must occur in consecutive readings of said variable before said control moves to said higher surge control limit.
 6. The compressor system as set forth in claim 1, wherein said control moves back to said low surge control limit once the change in the variable no longer exceeds the threshold.
 7. The compressor system as set forth in claim 1, wherein said change in said variable is negative, and must be lower than a predetermined threshold for said control to switch to said higher surge control limit.
 8. The compressor system as set forth in claim 1, wherein said surge control valve also incorporates a bleed valve, with said bleed valve controlling the flow to the downstream use.
 9. The compressor system as set forth in claim 8, wherein said bleed valve and said surge control valve are incorporated into a single valve.
 10. The compressor system as set forth in claim 8, wherein said bleed valve and said surge control valve are two separate valves.
 11. An aircraft air supply system including: an auxiliary power unit powering a load compressor, said load compressor delivering air to a use on an aircraft; said load compressor having an air inlet and an outlet, a surge control valve positioned on said outlet, said surge control valve serving to selectively pass air to the use, or to be dumped; and a control for said load compressor and said surge control valve provided with a variable indicative of a movement of said compressor to transient operation, said control further being provided with a low surge control limit, and said control being provided with a higher surge control limit, said control switching to use of said higher surge control limit if the variable being monitored is seen to change beyond a particular threshold, and said low and higher surge control limits being utilized to control the opening of said surge control valve.
 12. The aircraft air supply system as set forth in claim 11, wherein said variable is a dynamic head pressure of said load compressor.
 13. The aircraft air supply system as set forth in claim 12, wherein said dynamic head pressure is inversely proportional to a discharge pressure of said load compressor.
 14. The aircraft air supply system as set forth in claim 11, wherein said change in said variable must occur in consecutive readings of said variable before said control moves to said higher surge control limit.
 15. The aircraft air supply system as set forth in claim 11, wherein said control moves back to said lower surge control limit once the change in the variable no longer exceeds the threshold.
 16. The aircraft air supply system as set forth in claim 11, wherein said change in said variable is negative, and must be lower than a predetermined threshold for said control to switch to said higher surge control limit.
 17. The aircraft air supply system as set forth in claim 11, wherein said use on the aircraft includes both a passenger cabin air supply, and the supply of air to a main gas turbine engine on the aircraft.
 18. The aircraft air supply system as set forth in claim 11, wherein said surge control valve also incorporates a bleed valve, with said bleed valve controlling the flow to the downstream use.
 19. The aircraft air supply system as set forth in claim 18, wherein said bleed valve and said surge control valve are incorporated into a single valve.
 20. The aircraft air supply system as set forth in claim 18, wherein said bleed valve and said surge control valve are two separate valves.
 21. A method of operating a load compressor comprising: controlling a surge control valve by providing a variable indicative of movement of a compressor to transient operation, said control further being provided with a low surge control limit, and said control being provided with a higher surge control limit, said control switching to use of said higher surge control limit if the variable being monitored is seen to change beyond a particular threshold, and one of said low and higher surge control limits being used to open said surge control valve when said one of the low and higher surge control limit is exceeded. 